Recently, printed circuits have been required to have an elevated wiring density and an elevated functioning due to development of increasingly small-sized and enhanced performance electronic instruments. Accordingly, the material of the substrate thereof needs to have the smallest possible dielectric constant, the highest possible insulating resistance value, and a good heat resistance. A substrate which satisfies these requirements in polyimide. A polyimide resin is frequently used as a material for FPC and TAB tapes. In general, a copper polyimide substrate having a copper metal layer on one or both surfaces of a filmy polyimide resin is commonly used.
For forming a copper layer on a polyimide resin film, a so-called lamination method has heretofore been employed in which a polyimide resin film is laminated to a copper leaf via an adhesive. In the method, however, since the adhesive has on adverse influence on the insulating property and heat resistance of the substrate, a different method has become employed in which a copper layer is directly formed on a polyimide film by sputtering, ion-plating, vacuum deposition plating or electroless plating.
However, where a copper-polyimide substrate obtained by directly forming a copper layer on the surface of a polyimide film is left in a high temperature atmosphere for a long period of time, there occurs a dangerous problem that the adhesion strength at the interface between the copper layer and the polyimide resin film decreases, such that the layer peels off from the film. After various investigations concerning this problem, it has been found that the lowering of the adhesion strength is caused by diffusion of copper to the polyimide film at the interface between the copper layer and the polyimide film. As a method of preventing the diffusion of copper, an interlayer metal barrier layer an interlayer polyimide film prior to forming the copper layer on the film.
Heretofore, formation of a metal interlayer of nickel, cobalt or the like between a polyimide resin film and a copper layer prior to formation of the copper layer on the polyimide film has been proposed for the purpose of preventing a decrease in the adhesiveness of copper to the film due to thermal shock applied to the film substrate by soldering, for example, as shown in JP-A 63-286580 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"), and U.S. Pat. No.4,725,504, 4,868,071 and 4,950,553. It is said that the thickness of the metal interlayer formed for the purpose must be 0.15 um or more.
Where a copper-polyimide substrate is used in manufacturing FPC and TAB tapes, formation of plural branched fine copper wires on the substrate by a subtractive process, a semi-additive process or a full-additive process is necessary. For instance where formation of such copper wires on the substrate is effected by a subtractive process, a photo-resist is coated on a copper-polyimide substrate having a electroless plated copper layer as formed on a polyimide film or on a substrate additionally having an electroplated copper layer as formed on the copper layer, a photo-mask having a determined lead pattern is closely attached thereto, the substrate is then exposed, developed and etched, whereby the exposed metal layer area is dissolved and removed, and the remaining photo-resist is thereafter peeled and removed to finally form the intended copper wires.
Therefore, in formation of the wires by the process, if a copper-polyimide substrate having a nickel or the like metal interlayer as formed between the polyimide resin film of the substrate and the copper layer in a thickness of 0.15 .mu.m or more, for example, by the process described in the above-mentioned JP-A 63-286580 and U.S. patents is used, the etching speed differs between the copper and the metal of a different kind of the interlayer, such as nickel or cobalt. In general, since the dissolution speed of the metal of a different kind is lower than that of copper, the interlayer of the metal of a different kind remains on the surface of the polyimide film in formation of copper wires by etching to cause a decrease in the insulating resistance between the wires. If the metal of the interlayer is desired to be completely dissolved and removed, the copper layer would be over-etched so that the side walls of the wire,,s would be dissolved too much, causing deformation of the formed wires. As a result, copper wires of the desired shape will not be obtained.
In the case of employing a semi-additive process in formation of copper wires, the means of forming copper wires by etching is not basically different from the above-mentioned subtractive process, though the procedure of forming copper wires somewhat differs between them. Therefore, also in this case, any satisfactory result could not be obtained because of the same reasons as above.
Accordingly, where FPC and TAB tapes are manufactured by the use of a copper-polyimide substrate having an interlayer of a metal of a different kind, such as nickel or cobalt, addition of a step of selectively etching only the metal of a different kind, such as nickel or cobalt, to the process of manufacturing them is necessary for the purpose of eliminating the above-mentioned problem. However, addition of such a step is troublesome and makes the process complicated, which is economically unfavorable. Since the copper-polyimide substrate having an interlayer of nickel, cobalt or the like metal, as manufactured by the process as disclosed by the above-mentioned JP-A 63-286580 and U.S. patents, is hydrophilicated on the surface of the polyimide film, the surface of the polyimide film absorbs the ambient moisture and swells when the substrate is stored in a high-temperature and high-humidity atmosphere for a long period of time, such that the adhesion strength of the film to the copper layer is thereby lowered, and results in the copper layer being peeled off from the film.